FIG. 3 is a sectional side elevational view illustrating an arrangement of a conventional fluid control valve configured to change a flow channel of fluid by displacing a spool within a main valve body, thereby controlling supply of fluid to a lift cylinder and discharge of fluid, and FIG. 4 is a sectional view taken on line X-X of FIG. 3.
Specifically, the fluid control valve BB illustrated includes: a main valve body 1 having a sleeve 6 to which a suction port 3 for sucking hydraulic fluid, a discharge port 4 for discharging hydraulic fluid and a cylinder port 5 for delivering hydraulic fluid to a lift cylinder C are connected, an electromagnetic solenoid valve 7 for opening and closing the flow channel between the cylinder port 5 and the discharge port 4, and an operate check valve 8 located between the sleeve 6 and the cylinder port 5 and having a lift lock poppet 9 therewithin for opening and closing the flow channel therebetween; and a spool 2 expandably and retractably fitted in the sleeve 6 and capable of selectively assuming a lifting position for providing communication between the cylinder port 5 and the suction port 3 to form a flow channel of hydraulic fluid, a lowering position for providing communication between the cylinder port 5 and the discharge port 4 to form a flow channel of hydraulic fluid, and a neutral position for shutting off the flow channels between these ports.
The spool 2 is formed with first to third annular grooves 2a to 2c while being connected to a non-illustrated operating lever via a non-illustrated link mechanism, so as to be capable of selectively assuming the lifting position, the lowering position and the neutral position.
Around the sleeve 6 of the main valve body 1, there are formed a high-pressure passage 11 connected to the suction port 3, a tank passage 12 connected to the discharge port 4, a hydraulic fluid delivery passage 13 which communicates with the high-pressure passage 11 via the first annular groove 2a when the spool 2 assumes the lifting position and which communicates with the tank passage 12 via the first annular groove 2a when the spool 2 assumes the lowering position, and a hydraulic fluid discharge passage 14 which communicates with the tank passage 12 via the third annular groove 2c when the spool 2 assumes the lowering position. The electromagnetic solenoid valve 7, which is located above the hydraulic fluid discharge passage 14, is always open during operation. The hydraulic fluid discharge passage 14 extends from the operate check valve 8 to the sleeve 6.
The operate check valve 8 includes the aforementioned lift lock poppet 9, and a plug 8a accommodating the lift lock poppet 9 therein. The lift lock poppet 9 is movable between an open position for providing communication between the cylinder port 5 and the hydraulic fluid delivery passage 13 and a shutoff position for shutting off the communication between the cylinder port 5 and the hydraulic fluid delivery passage 13. The lift lock poppet 9 has a back-pressure chamber 9a provided therewithin and an orifice 9b located at the outer periphery thereof for providing communication between the back-pressure chamber 9a and the outside. The back-pressure chamber 9a communicates with the hydraulic fluid discharge passage 14. A cylinder passage 15 is provided between the operate check valve 8 and the cylinder port 5. When the lift lock poppet 9 is in the open position, the cylinder passage 15 is allowed to communicate with the hydraulic fluid delivery passage 13. The communication between the cylinder passage 15 and the hydraulic fluid delivery passage 13 is shut off when the lift lock poppet 9 is in the shutoff position.
The high-pressure passage 11 is provided with a load check poppet 20 and a spring 21 for preventing a carriage from sinking due to hydraulic fluid flowing backward from the lift cylinder C to a high-pressure pump just after a carriage lifting operation has started and just before the carriage lifting operation terminates. (See Registered Japanese Utility Model Publication No. 3115605 for example.) Description will be made of operations of respective portions of the fluid control valve BB in moving a lift up and down.
In moving the lift up, the spool 2 is moved rightwardly from the position illustrated (i.e., neutral position). Accordingly, the first annular groove 2a shifts rightwardly to make the high-pressure passage 11 and the hydraulic fluid delivery passage 13 communicate with each other via the first annular groove 2a. As a result, hydraulic fluid from the high-pressure pump pushes the load check poppet 20 open and flows into the hydraulic fluid delivery passage 13 through the high-pressure passage 11, thus applying a high hydraulic fluid pressure to the lift lock poppet 9. When the lift lock poppet 9 is moved upwardly in FIG. 3 by the hydraulic fluid pressure, communication is provided between the hydraulic fluid delivery passage 11 and the cylinder passage 15 to allow hydraulic fluid to be fed to the cylinder port 5 via the cylinder passage 15 (see hydraulic fluid flow (h)). Thus, high-pressure hydraulic fluid is supplied into a bottom chamber C1 of the lift cylinder C, thus moving the lift up. With the lift in this moved-up position, when the spool 2 is moved back into the neutral position, the communication between the hydraulic fluid delivery passage 13 and the high-pressure passage 11 and the communication between the hydraulic fluid delivery passage 13 and the tank passage 12 are shut off. Since the communication between the hydraulic fluid discharge passage 14 and the tank passage 12 remains shut off at that time, the lift cylinder C is kept in the condition described above. Then, the lift lock poppet 9 moves downwardly in FIG. 3 to shut off the communication between the hydraulic fluid delivery passage 13 and the cylinder passage 15.
In moving the lift down, on the other hand, the spool 2 is moved leftwardly. By so doing, the hydraulic fluid discharge passage 14 is allowed to communicate with the tank passage 12 and the discharge port 4 via the third annular groove 2c. That is, the poppet back-pressure chamber 9a of the operate check valve 9 is allowed to communicate with the tank passage 12 to produce a primary hydraulic fluid flow (i). The primary hydraulic fluid flow (i) causes a pressure difference to occur between the cylinder passage 15 and the poppet back-pressure chamber 9a. In turn, the pressure difference causes the lift lock poppet 9 to open by moving upwardly, so that hydraulic fluid in the bottom chamber C1 of the lift cylinder C flows into the tank passage 12 via the hydraulic fluid delivery passage 13 and the first annular groove 2a of the spool 2 to produce a secondary hydraulic fluid flow (j), thereby causing the lift cylinder C to perform a lowering operation. With the lift cylinder in the lowered position, when the spool 2 is moved back into the neutral position, the communication between the hydraulic fluid discharge passage 14 and the tank passage 12 is shut off. As a result, the pressure difference between the cylinder passage 15 and the back-pressure chamber 9a disappears, which allows the lift lock poppet 9 to move into the shutoff position by the spring 9d. Thus, the communication between the cylinder passage 15 and the hydraulic fluid delivery passage 13 is also shut off.
When the forklift truck is at rest, the electromagnetic solenoid valve 7 is closed because its solenoid is not applied with a voltage. Therefore, the primary hydraulic fluid flow (i) is not produced even when the spool 2 is moved leftwardly. Accordingly, the lift lock poppet 9 fails to open and, hence, the lift fails to move down.